1. Field of the Invention
The present invention relates to an optical diffusion film that is laminated on an upper surface or lower surface side of a liquid crystal panel in an optical display device such as a liquid crystal display unit.
2. Description of the Related Art
In an optical display device such as a liquid crystal display unit, a plurality of optical diffusion films are laminated on the top and bottom of a liquid crystal panel and used.
For example, International Publication No. WO 99/42861 discloses an optical diffusion film that includes structural members that vary in height along their length. In the optical diffusion film, the variations have a nominal period of less than forty times the nominal height of the structural members.
In the conventional optical diffusion film described above, although variations are provided in the lenticular heights, the variations are inadequate and do not have a diffusion effect. Consequently, there has been a problem regarding the occurrence of interference fringes or a wet-out phenomenon between laminated optical diffusion films or between an optical diffusion film and a liquid crystal panel, and there has also been a problem with a cut-off that is caused by a rapid change in brightness within a viewing angle. There has also been a problem of a decrease in a display quality such as occurrence of moiré that is caused by the relationship between a size of a light transmissive basic unit of the display panel and the periodicity of a lens pitch.
Further, in a conventional optical display device such as a liquid crystal display unit, it has been necessary to use a diffusion sheet as a protective sheet of a surface of an upper portion structure of an optical diffusion film, and to perform a diffusive matte treatment on a surface on an opposite side of a structured surface. However, forming these separate members or performing the matte treatment work requires labor man-hours and increased costs.